1. Field of the Invention
The present invention relates to an injection controlling method for an injection molding machine and, more particularly, to an injection controlling method suitable for molding an article having a small thickness.
2. Description of the Related Art
Motor-driven injection molding machines have increasingly been used in recent years, in which a servo motor is used as a substitute for a hydraulic actuator. Operations of such injection devices using servo motors are summarized below.
A screw is rotated with a servo motor for screw rotation in a plasticization/metering process. The screw is located within a heating cylinder. A resin is fed from a hopper to a rear portion of the screw in the heating cylinder. Rotation of the screw melts and advances a certain metered amount of the supplied resin to a nose portion of the heating cylinder. During this time, the screw is retracted due to pressure (back pressure) of the molten resin stored in the nose portion of the cylinder.
An injection shaft is directly connected to a rear end portion of the screw. The injection shaft is rotatably supported by a pressure plate through a bearing. The injection shaft is driven in the axial direction by a servo motor for injection that is supported on the pressure plate. The pressure plate moves forward and backward along guide bars in response to the operation of the servo motor for injection through a ball screw. The above-mentioned back pressure of the molten resin is detected by using a load cell and is controlled with a feedback control loop, as will be described more in detail below.
Then, in a filling process, the pressure plate is advanced by driving the servo motor for injection. The nose portion of the screw serves as a piston to fill a mold with the molten resin.
The molten resin fills the space within the cavity at the end of the filling process. At this point, the control mode for the advancing motion of the screw is switched from a velocity control mode to a pressure control mode. This switching is referred to as a xe2x80x9cV (velocity) to P (pressure) switchingxe2x80x9d and affects the quality of resultant molded articles.
Following the V-P switching, the resin within the mold cavity is allowed to cool under a predetermined pressure. This process is referred to as a dwelling process. In this dwelling process, the pressure of the resin is controlled in the feedback control loop as in the above-mentioned back pressure control.
Then, the injection device returns to the plasticization/metering process after the completion of the dwelling process. A mold clamping device performs an eject operation for ejecting a solid product out of the mold in parallel with the plasticization/metering process. In the eject operation, the mold is opened to remove the solidified product from the mold by an ejector mechanism, and then the mold is closed for the filling process.
As well known in the art, injection control is performed in the filling process. The injection control should be changed depending on the thickness of an article. If the injection control for a molded product having a large thickness is applied to the molding of a thin molded article, a number of problems will arise. For example, with the application of such injection control to the molding of a thin molded article, an excessive pressure exerted to the molded article may generate flash and/or residual stress, which may result in a defective or flawed article. Therefore, the pressure exerted to the molded article should be reduced. However, the conventional injection control methods cannot effectively reduce the pressure suddenly because the maximum traveling velocity or the acceleration of the screw is smaller in the dwelling process than in the filling process.
Therefore, an object of the present invention is to provide an injection control method which is improved to ensure weight and dimensional stability of a molded article between adjacent shots in molding a thin molded article.
The present invention is applied to an injection molding machine which carries out a molding cycle including a resin plasticization/metering process and a resin filling process.